There is old history of chemical warfare in Syria

The earliest archaeological evidence of chemical weapons in Syria is 1,700 years old.

The first way that people gassed their enemies was with a mixture of sulfur and pitch in combination with fire. Archaeologist Simon James said the poisonous gas had been used around 256 AD during a siege on the city of Dura-Europos.

The site had first been excavated in 1920 and later in the next decade, 20 bodies of Roman soldiers had been found in one of the counter-mines that had been dug in order to be able to access the tunnels.

A theory suggested that the soldiers had died because of the collapse of the tunnel but James presented proof that those people had actually been gassed. Residue composed of pitch and yellow sulfur crystals had been found in a jar near the bodies.

James explained saying "the Sasanians placed braziers and bellows in their gallery, and when the Romans broke through, added the chemicals and pumped choking clouds into the Roman tunnel."

Alexander's army had also been struck by a sulfur and pitch fireball in 327 BC but researchers believe there is no way to tell if it was by accident or done on purpose.